The contour sharpness of a video image in the direction of the lines can be increased by aperture correction of the image recording video signal, by twice differentiating the video signal; a variable proportion of the second differential quotient of the video signal, corresponding to the correction signal, is added to the video signal with reverse polarity. It has also been proposed--see U.S. Pat. No. 4,030,121--`to increase the steepness of flanks of video signal signal jumps by applying to a four-quadrant multiplier a signal corresponding to the first differential quotient of the video signal and, after rectification, applying a signal corresponding to the second differential quotient of the video signal after amplitude limiting. The video signal corresponding to the second differential quotient acts as a switching signal to invert the first half of the video signal corresponding to the first differential quotient in time. A similar circuit is also described in German Published Patent Application DE-AS No. 22 37 769, in which, however, a signal corresponding to the square of the first differential of the video signal is generated which is inverted by a switching signal corresponding to the second differential quotient of the video signal, temporally, in the first half.
The circuits described provide correction signals which are suitably additively superimposed on the video signal to be corrected.
It has been found that the switch-over effected by the inversion generates strong harmonics which may lead to cancellation of higher frequency portions of the video signal. It has also been found that they lead to increased color ambiguities and cross-talk.